Dielectric measurements (thermally stimulated depolarisation currents and broadband dielectric spectroscopy) have been performed near the glass transition to study the glass transition on the odd non-symmetric liquid crystal (LC) dimers of the series -(4-cyanobiphenyl-4'-oxy)--(1-pyreniminebenzylidene-4'-oxy) alkanes (CBOnO.Py) with n ranging from 3 to 9. A previous study [S. Diez-Berart et al., Materials 8 (2015) 3334] carried out in CBO11O.Py showed the presence of two glass transition temperatures, attributed to different molecular motions of the terminal groups. The study performed allows us to analyse the molecular dynamics in the rest of the series and determine the role played by the flexible spacer. Parallel and perpendicular orientations of the molecular director with regards to the probe electric field have been investigated. The low and intermediate observed relaxations are explained in the framework of Stocchero's theoretical model [M. Stocchero, J. Chem. Phys. 121 (2004) 8079] for the dielectric behaviour of non-symmetric LCs dimers, as independent end-over-end rotations of each terminal semi-rigid unit. As the length of the spacer chain in the series of compounds decreases, the different relaxations become progressively more coupled at the glass transition. Numerical simulations of the calorimetric response from the obtained kinetic parameters show good agreement with experimental behaviour.

Two commercially available polyetherimides, Ultem 1000 and Ultem 5000, have been studied by means of Dynamic Electrical Analysis. Results show that at temperatures above the glass transition dielectric response is highly influenced by space charge. Obtained data is analyzed using the electric modulus formalism. The real part of the conductivity is conveniently described by a sublinear power law dependency (¿n with n(c) 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or
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In the present work, the nematic glassy state of the non-symmetric LC dimer -(4-cyanobiphenyl-4-yloxy)--(1-pyrenimine-benzylidene-4-oxy) undecane is studied by means of calorimetric and dielectric measurements. The most striking result of the work is the presence of two different glass transition temperatures: one due to the freezing of the flip-flop motions of the bulkier unit of the dimer and the other, at a lower temperature, related to the freezing of the flip-flop and precessional motions of the cyanobiphenyl unit. This result shows the fact that glass transition is the consequence of the freezing of one or more coupled dynamic disorders and not of the disordered phase itself. In order to avoid crystallization when the bulk sample is cooled down, the LC dimer has been confined via the dispersion of -alumina nanoparticles, in several concentrations.

Charge distribution and transport have been investigated in LDPE films with different humidity content under electric fields up to 130 MV/m. Pulsed electroacoustic measurements showed that, as water content increases, positive charge packets formation in the anode is enhanced and they propagate toward the cathode with higher transit speeds. Fits of surface potential decay measurements showed that charges in dry samples are injected directly into the volume, but the presence of moisture generates new trap centers in the surface of the material. This new trap level causes a charge accumulation on the surface, that gradually passes into the bulk. The observed behavior in development and propagation of charge packets are explained according to these results. Thermally stimulated depolarization current measurements showed a non-distributed relaxation associated to the new trap levels on the surface of the wet samples.

In order to study charge trapping and transport in polyimide, we have submitted samples of Kapton HN to corona charging and measured its surface potential decay with an electrostatic voltmeter. We propose a two terms mobility to explain the experimental data: a non-dispersive contribution based on Toomer and Lewis model added to a dispersive process, which is associated with the disordered structure of the material. The non-dispersive model alone did not fit well to the data for short times, but our assumption makes the theoretical expression fit successfully to the experimental data. Some important parameters related to the charge transport properties of the material are determined and discussed.

In order to study charge trapping and transport in polyimide, we have submitted
samples of Kapton HN to corona charging and measured its surface potential decay
with an electrostatic voltmeter. We propose a two terms mobility to explain the
experimental data: a non dispersive contribution based on Toomer and Lewis model
added to a dispersive process, which is associated with the disordered structure
of the material. The non-dispersive model alone did not fit well to the data for
short times, but our assumption makes the theoretical expresion fit succesfully to
the experimental data. Some important parameters related to the charge transport
properties of the material are determined and discussed.

The analysis by TSDC of a large variety of
cables subjected to long annealing times, has resulted in spectra that sometimes show an oscillation of the current peak observed at 99°C. This process has been observed
in some cables only and it is scarcely reproducible. We present an explanation of this behavior based on the formation
of charge packets and some other assumptions:
charge injection occurs mainly from the negative electrode, and chemical components that act as trapping centers are diffused continously from the semiconducting layers
into the bulk during annealing. When the field is applied the negative electrode injects charge that get trapped in
these centers, so that a virtual electrode is formed. The electric field created by this layer results in a decrease of the field in the gap between the injecting and virtual electrodes, preventing new charge injection. On the other hand, during polarization the virtual electrode shifts through the insulation bulk towards the opposite electrode.
This layer that acts as a virtual electrode becomes neutral by means of recombination when it reaches the opposite electrode, so that the electric field in the region between
the electrodes recovers its initial value and the whole process happens again cyclically. This model can explain the
observed TSDC oscillating spectra of annealed samples.
It can also explain why the oscillations can not be observed in most cases.

Three experimental midvoltage XLPE cables, C2, C3 and C4, rated respectively as 'good', 'very good' and 'bad' in perforation tests, are studied. All these cables have been systematically measured by PEA as produced and after being annealed at 90ºC and 120ºC up to 672 hours.
Measured internal charge of cable C4 at least doubles that of cables C2 and C3. Evolution with annealing show as well differences in the reticulation process carried out during cable C2 and C3 preparation, which can explain why cable C3 performs better than C2 in perforation
tests. Infrared spectroscopy measurements (IR) showed differences in components injected from the external semiconducting layer (SC) into the isolation during annealing. These results explain other observed differences in PEA measurements with regard of the SC type of each cable. To sum up, combination of PEA and IR measurements are a useful tool in understanding charge relaxation processes and XLPE cable performance.